1. Field of the Invention
This invention relates to an apparatus for the electric level control of a developing solution contained in a storage tank and used for developing two-component diazo-type material according to a semi-dry developing process.
2. Description of the Prior Art
Diazo printers with developing stations operative in accordance with the semi-dry development process are equipped with roll-application means which include means for supplying developing solution to the two-component diazo-type sheet material. The roll-application means comprise mainly an applicator roll, a press roll and a back-up roller which forces the press roll against the applicator roll.
A problem occurs in the use of roll-application means of this type in that the applicator roll which is usually covered with a rubber coating, has a relatively delicate surface, and damage to this surface often prevents uniform development. A dry running of the applicator roll under the compressive load exerted by the press roll and the back-up roller must, consequently, be avoided, because dry running results in a sudden strong increase of the frictional and flexing forces at the applicator roll which may damage or even destroy the surface of the applicator roll. It is, therefore, necessary to always have sufficient developing solution for moistening the applicator roll. A lack of developing solution may be caused by normal consumption, by suddenly occurring leaks or by a breakdown of a feed pump which conveys the developing solution from a larger supply container to the storage tank of the developing device.
The level of the developing solution in the storage tank of the developing device may generally be monitored by controlling means provided with a bubble tube or a float or by optical or electromagnetic control systems. In the present case, however, these known devices do not provide reliable monitoring, since foaming on the surface of the developing solution caused in particular by pumping the solution into the storage tank, interferes with these level control monitoring devices. Further, prior art control means, in particular those functioning according to optical and electromagnetic systems are expensive, especially if different signals are to be generated when the developing solution exceeds or falls below different level heights.